1. Field of the Invention
The present invention relates to a method of preparing a platinum catalyst. More specifically, the present invention relates to a method of preparing an improved platinum catalyst for use in fuel cell electrodes.
2. Description of the Related Art
Spurred on by increasing oil prices and growing concerns over environmental pollution, the use of fuel cells has sparked global interest as an alternative to fossil fuel and combustion technologies. Fuel cells are attractive for a number of reasons, e.g. low pollution, high energy efficiency, fuel flexibility, high quality power output, quick response to load fluctuations, excellent heat recovery characteristics, quiet operation, etc. Their high energy efficiency and low pollution partly derive from the use of a clean fuel source, e.g. hydrogen, methanol, etc.
Platinum catalysts are often employed in fuel cell electrodes since they can increase the fuel cell's power density. However, the amount of natural platinum deposits is in limited supply, and its use in fuel cell electrodes quite cost-prohibitive. Accordingly, efforts are being made toward developing a catalyst for fuel cell electrode, which uses a lower quantity of platinum yet retains relatively high energy efficiency as compared to the conventional fuel cell electrode catalyst.
The conventional platinum catalyst used in fuel cell electrode requires a high quantity of platinum to be loaded on carbon supports. Typically, 20 weight parts of platinum particles are loaded on 100 weight parts of carbon supports. However, when too much platinum is loaded onto carbon supports, the platinum particles tend to agglomerate and form larger particles. This in turn reduces the specific surface area of the catalyst and lowers the overall catalytic activity. In other words, a catalyst with larger platinum particles has lower catalytic activity than a catalyst with smaller particles even when the amount of platinum deposited and amount of carbon supports are held constant. Catalyst supports that are currently in commercial use have a relatively high average specific surface area, e.g. 250 m2/g. In conventional catalysts, about 20 weight parts of platinum can be uniformly dispersed on 100 weight parts of carbon supports. However, agglomeration of platinum particles results when more than 20 weight parts of platinum particles are loaded onto 100 weight parts of supports.
One attempt to solve the agglomeration problem described above is found in Korean Unexamined Patent Publication No. 2002-84372, which proposes using a mesoporous carbon substance with high specific surface area as a catalyst support. This substance can prevent platinum particles from agglomerating by enabling a large number of platinum particles to be uniformly dispersed on its surface, so as to form a catalyst with higher catalytic activity for applications in fuel cell electrodes.
To form this mesoporous carbon substance, carbon precursors such as carbohydrate or carbon polymer precursors are forced to permeate into pores of a silica template, i.e. a structure with uniformly-sized pores. The silica template, having been impregnated with carbon precursors, is dried and polymerized using an acid catalyst. The resultant composite is then subjected to a high temperature for thermal decomposition to yield a silica-carbon composite. The silica template is removed from said composite using either a strong base, e.g. sodium hydroxide (NaOH), or a strong acid, e.g. hydrofluoric acid (HF). The resultant structure is then rinsed with ethanol and water and filtered to produce a mesoporous carbon substance with large uniformly-sized pores. The final steps in preparing a platinum catalyst for applications in fuel cell electrodes involve treating the surface of the mesoporous carbon and loading small uniformly-sized platinum particles thereupon.
While the mesoporous carbon substance of the prior art can support a high quantity of platinum, its preparation and use in fuel cell electrodes present several problems. For instance, the pores of the platinum catalyst as prepared by the conventional method become clogged during the platinum loading step. This can lower catalytic activity, which in turn detracts from fuel cell performance. In addition, the conventional method of preparing a platinum catalyst lacks a high temperature thermal treatment step, causing sintering, i.e. agglomeration of metal particles on the catalyst, to occur as a result of exposure to heat generated during fuel cell operation. The many labor-intensive steps and long processing time are additional disadvantages associated with the conventional method of preparing a platinum catalyst using the mesoporous carbon substance. In light of the above, there is a need in the art to overcome these problems associated with preparation of a platinum catalyst for use in fuel cell electrode.